Although some argue the idea/expression dicotomy in US Copyright law is a bad thing and others argue it no longer exists, Ip's What's Up would like to add the idea/expression dicotomy to the list of good things about copyright law.
The essence of the idea/expression dicotomy is that an author can copyright his own expression of something but not the underlying idea itself. For example, if I were to paint a picture of an apple, I would have copyright over that particular apple picture but not over all paintings of apples. The idea to paint the apple is not protected by copyright.
This dicotomy can sometimes be tricky, but it is still a plus in the US copyright system. It allows authors to create freely from their surroundings and influences in their lives without risking infringing on someone else's work. Some ideas are common and shared among communities, others may be thought of by more than one person. Different authors may both create television series about a space ship and its crew of different species traveling through space, encountering aliens, visiting strange places and engaging in battles. Some viewers may prefer this idea in a fairly serious drama expression, other viewers may prefer the idea in the form of a relaxed, and sometimes vulgar, comedy. The idea/expression dicotomy allows authors to express this space crew idea in both these forms and many more.
By protecting only the expression of the idea, copyright law allows a greater amount of creations. Authors produce different works based on similar things. Members of society have the ability to choose the expression they like best. Authors get more freedom. Society gets more choices. That's a happy outcome.